[ccan] [PATCH] rszshm: New module

[Rusty Russell]

Dan Good <dan at dancancode.com> writes:
> rszshm - resizable pointer-safe shared memory
>
> Signed-off-by: Dan Good <dan at dancancode.com>

Nice!  This is very similar to what antithread does, but pulled out
into a module of its own.

> On Linux boxes it can be seen that the used addresses clump
> + * at either end of the address range.  rszshm tries to mmap in the middle
> + * of the address range, and checks if the requested address matches the
> + * returned address.  If not, additional addresses are tried.  Once mapped,
> + * the address is recorded to a header in the file.  Another process reads the
> + * header and requests the saved address from mmap.  If the returned address
> + * matches, work proceeds.  While the defaults provide a propitious search,
> + * all the search parameters may be specified.

In antithread, I used the following heuristic:

/* We add 16MB to size.  This compensates for address randomization. */
#define PADDING (16 * 1024 * 1024)

	p->pool = mmap(NULL, size+PADDING, PROT_READ|PROT_WRITE, MAP_SHARED, fd,
		       0);
	if (p->pool == MAP_FAILED)
		goto fail_free;

	/* Then we remap into the middle of it. */
	munmap(p->pool, size+PADDING);
	p->pool = mmap((char *)p->pool + PADDING/2, size, PROT_READ|PROT_WRITE,
		       MAP_SHARED, fd, 0);
	if (p->pool == MAP_FAILED)
		goto fail_free;

The 16MB probably needs to be much larger for 64-bit machines, though
(and I didn't support resize nor renegotiation, as that's an issue if
you want to be able to create new antithreads dynamically...)

Cheers,
Rusty.